Radiation diffracting materials based on crystalline colloidal arrays have been used for a variety of purposes. A crystalline colloidal array (CCA) is a three-dimensional ordered array of monodispersed colloidal particles, which may be composed of inorganic or organic materials. Colloidal dispersions of particles can form crystalline structures having lattice spacings that are comparable to the wavelength of ultraviolet, visible, or infrared radiation. These structures have been used for filtering narrow bands of selected wavelengths from a broad spectrum of incident radiation while permitting the transmission of adjacent wavelengths of radiation. Other CCAs have been used when fixed in a matrix as colorants, where the fixed array diffracts radiation in the visible spectrum. Such colorant CCAs exhibit a goniochromatic effect when applied to a substrate.
More recently, radiation diffracting materials have been used for producing images with unique optical effects. Such radiation diffracting materials have been proposed for use in security devices for authenticating documents or as security applications. Secure documents, such as bank notes, bonds, checks, and letters of credit, as well as legal documents and identification documents now frequently carry optically active devices, some exhibiting an angularly dependent colored reflection. The use of radiation diffracting materials in security devices has been relatively limited and generally restricted to simple angular dependent colored reflection, which is determined upon viewing the article bearing the device at varying angles.